In a standard internal-combustion motor-vehicle engine flow into and out of the combustion chambers formed above the pistons is typically through respective intake and exhaust ports that are each closed during the compression and power strokes by a respective poppet or lift valves and respectively open during the intake and exhaust strokes. The intake ports are all connected to an intake manifold and the exhaust ports are all connected to an exhaust manifold. Each poppet valve is formed as a plug that is accurately machined to fit tightly in the respective port and that can be lifted, invariably by displacement into the chamber toward the pressurized side, to allow flow through the respective port. Such poppet valves are mounted on rods having tappet ends that ride on a camshaft that opens and closes them synchronously as needed by the type of engine.
The advantage of a lift or poppet valve is that it can withstand enormous pressure and extremes of heat. Thus it is ideal for use in a high-pressure hot spot in the wall of a cylinder of an internal-combustion engine. The disadvantage of such a valve is that it is fairly expensive to manufacture, due mainly to the high-quality heat-resistant metal it must be made of and the high accuracy with which it must fit in its seat. In addition two such valves must be provided for each cylinder, one to connect it to the exhaust manifold that carries away spent gases during the exhaust stroke and one to the intake manifold that supplies air during the intake stroke.
Hence attempts have been made to employ other valve types. U.S. Pat. Nos. 5,081,966, 5,154,395, 5,191,863, and 5,205,251 describe various systems employing a sleeve valve. Such a valve has a cylindrical sleeve formed with a port and rotated so that this port aligns with another port in a wall closely surrounding the sleeve. In such a valve the element moves parallel to its seat, as opposed to perpendicular to it like the poppet valve, and always moves in the same direction. Furthermore the sleeve can move continuously so that the wasted energy of a reciprocating movement of the valve body is avoided and certain other advantages are obtained. It is, however, very difficult to make such a valve sturdy enough to withstand long-term high-pressure use. Furthermore machining such a valve to form the very tight seal required for a combustion chamber of an internal combustion engine makes this alternative very costly.